1. Field of the Invention
The present invention relates to devices and methods which are used to close septal openings. In particular, this invention is directed to devices and methods which are used to close a patent foramen ovale (PFO) in the septum between the left atrium and right atrium.
2. Background Information
A PFO, illustrated in FIGS. 1 and 2, is a persistent, one-way, usually flap-like opening in the wall between the right atrium and left atrium of the heart. Because left atrial (LA) pressure is normally higher than right atrial (RA) pressure, the flap usually stays closed. Under certain conditions, however, right atrial pressure can exceed left atrial pressure which creates the possibility that blood could pass from the right atrium to the left atrium and allow blood clots to enter the systemic circulation. It is desirable that this circumstance be reduced.
The foramen ovale serves a desired purpose when a fetus is gestating in utero. Since blood is oxygenated through the umbilical chord, and not through the developing lungs, the circulatory system of a heart in a fetus allows the blood to flow through the foramen ovale as a physiologic conduit for right-to-left shunting. After birth, with the establishment of pulmonary circulation, the increased left atrial blood flow and pressure results in functional closure of the foramen ovale. This functional closure is subsequently followed by anatomical closure of the two over-lapping layers of tissue the septum secundum 14 and septum primum 15. However, a PFO has been shown to persist in a number of adults.
The presence of a PFO is generally considered to have no therapeutic consequence in otherwise healthy adults. However, patients suffering a stroke or transient ischemic attack (TIA) in the presence of a PFO and without another cause of ischemic stroke paradoxical embolism via a PFO is considered in the diagnosis. While there is currently no proof for a cause-effect relationship, many studies have confirmed a strong association between the presence of a PFO and the risk for paradoxical embolism or stroke. In addition, there is good evidence that patients with PFO and paradoxical embolism are at increased risk for future, recurrent cerebrovascular events.
Accordingly, patients with an increased future risk are considered for prophylactic medical therapy to reduce the risk of a recurrent embolic event. These patients are commonly treated with oral anticoagulants, which have the potential for adverse side effects, such as hemorrhaging, hematoma, and interactions with a variety of other drugs. The use of these drugs can alter a person's recovery and necessitate adjustments in a person's daily living pattern.
In certain cases, such as when anticoagulation is contraindicated, surgery may be necessary or desirable to close the PFO. The surgery would typically include suturing a PFO closed by attaching the septum secundum to the septum primum. This sutured attachment can be accomplished with either an interrupted or a continuous stitch and is a common way a surgeon shuts a PFO under direct visualization.
Umbrella devices and a variety of other similar mechanical closure designs, developed initially for percutaneous closure of atrial septal defects (ASDs), have been used in some instances to close PFOs. These devices have the potential to allow patients to avoid the potential side effects often associated with anticoagulation therapies and the risks of invasive surgery. However, umbrella devices and the like which are designed for ASDs are not optimally suited for use as a PFO closure device.
Currently available designs of septal closure devices present drawbacks, including that the implantation procedure is technically complex. Additionally, there are not insignificant complication rates due to thrombus, fractures of the components, conduction system disturbances, perforations of heart tissue, and residual leaks. Many devices have high septal profile and may include large masses of foreign material which may lead to unfavorable body adaptation of a device. Since ASD devices are designed to occlude a hole, many lack anatomic conformability to the PFO flap-like anatomy. That is, when inserting an ASD into the heart to close a PFO, the narrow opening and the thin flap may form impediments to proper deployment. Even if an occlusive seal is formed, the device may be deployed in the heart on an angle which could leave some components not securely seated against the septum. Finally, some septal closure devices are complex to manufacture, which may result in lack of consistency in product performance.
Nitinol (an alloy of nickel and titanium) is known to be used in medical devices because of its biocompatablity and, especially, its unique properties. Nitinol is a member of a class of materials which exhibit shape memory characteristics. Specifically, nitinol has the ability to “remember” a shape and, after being deformed, will return to that shape once a certain temperature is reached. Hence, nitinol has been used to create medical devices that have a desired configuration in the body, e.g., a vena cava filter. The device is deformed into a delivery configuration (generally a reduced profile for delivery through a catheter). Once the device is delivered to the desired site, the nitinol changes configuration into the desired shape upon achieving a certain temperature. This is generally considered thermal responsive shape memory. Nitinol, and other materials, also exhibit superelastic shape memory. In this case, the nitinol can be restrained in a delivery configuration and then will return to the desired configuration as the device leaves the delivery catheter. Of course, some devices can be a combination of thermally induced shape memory and superelastic shape memory.
The present invention is designed to address these and other deficiencies of the prior art septal closure devices.